I'll echo dragonchild here.

"What if" about features that could come in late, and think of several ways to implement them. Maybe tinker with a few ideas, try them on toy problems, etc. Also put your happy hat on and think of things they'll never ask for, and how you might implement those too. (Trust me, some of those "nevers" will surprise you.)

There's no need to implement anything, as some choices will preclude others.

Think of Ender's Game, where Ender tells Bean to come up with wacky ideas for Battle Room. Near the end, they never know what to expect, and Ender and Bean always turn the scenario to their advantage.